1. Field of the Invention
This invention relates to the field of magnetic recording and reproducing systems for video signals and particularly to the type of system in which there is a polarity reversal of a certain portion of the signal at the end of each line interval of certain groups of line intervals.
2. The Prior Art
This system is related to the system described in copending U.S. application Ser. No. 492,330, filed July 26, 1974, and assigned to the assignee of the present application. In that system it was proposed to reduce the crosstalk interference of the low frequency portions of the video signals recorded on adjacent tacks in h-alignment by reversing the polarity of the chrominance components at the end of each line interval of alternate tracks but not to reverse the polarity at the end of line intervals during the remaining alternate tracks. As a result, the crosstalk component picked up during playback of each line interval would have a polarity that was either the same as or opposed to the polarity of the main, or desired, chrominance component signal. The chrominance components and cross-talk signals of successive line intervals were then combined in a comb filter, which is a type of filter that includes delay means and a subtractor circuit to combine, in opposite polarity, the signal applied to the delay means with the output signal of the delay means. The length of the delay is one line interval and so the chromiance signal for each line interval is combined in opposite polarity with the chrominance signal of the succeeding line interval. During the recording of alternate tracks the chrominance components of successive line intervals are recorded in opposite polarity so that when they are combined subtractively, the alternation in polarity cancels out and the chrominance components of successive line intervals then return to the same polarity and are added. However, the cross-talk signals, when passed through this same comb filter emerge in successively opposite polarities and so are cancelled out or, at least, are reduced.
For those tracks in which the successive line intervals of the chrominance signal are not reversed in polarity, switching means are provided in the playback apparatus to select, during alternate line intervals, chrominance components of opposite polarity so that, as applied to the comb filter, they do have the required successive opposite polarity condition. Again, the crosstalk signals reproduced along the desired chrominance signals of the latter tracks are affected by the same switching and comb filter arrangement so that they are cancelled or are at least substantially minimized.
The comb filter not only provides means for combining successive line interval signals, but also has a filtering effect that results in the substantially complete attenuation of signals having integral multiples of the fundamental frequency that is delayed by one cycle in passing through the delay means. In the case of delay means having a delay of one line interval, thus fundamental frequency is the basic line repetition frequency of the system.
The effect of inverting the polarity of chrominance components during successive line intervals is to produce a frequency offset. In the simplest terms, if a sine wave signal having the frequency f.sub.s of the chrominance signal carrier were periodically inverted at a repetition frequency f.sub.h, which may conveniently be understood to be the basic line repetition frequency, the resultant modified signal would not have the frequency f.sub.s any more but, by Fourier analysis, would be seen to be the combination of sinusoidal signals having frequencies f.sub.s +1/2(f.sub.h) and f.sub.s 1/2(f.sub.h). By choosing the frequency f.sub.s to be nf.sub.h +1/2(f.sub.h) where n is an integral number, the signal having the frequency f.sub.s could pass through the comb filter, but the signals having the frequencies f.sub.s .+-.1/2(f.sub.h) would not pass through the comb filter. This provides further separation of the desired signals, which may be the f.sub.s signal and its side bands spaced from it by mf.sub.h, from the undesired signals, which have frequencies f.sub.s .+-.1/2(f.sub.h) with side bands spaced mf.sub.h therefrom. The number m is an integer and usually is much smaller than n. The side bands of the desired signal interleave with side bands of the undesired, or crosstalk, signal and are all at frequencies to be separated from the undesired signal and its side bands by the frequency response of the comb filter as well as by the subtractive combination of successive line interval signals in the comb filter.
The circuit that achieves the desired switching of polarity of alternate line interval signals of the chrominance signal in the above-mentioned prior application may inadvertently and undesirably introduce a voltage offset. This is due to the fact that the signal of one polarity may have a certain DC axis and the signal of the other polarity may have a different axis so that when alternate line interval segments of these two signals are combined, the DC axes come through the switching operation as a square wave having a voltage magnitude equal to the difference in the DC axes. Even if the switched signal is passed through a filter to remove DC components, switching transients are still likely to remain. For example, if the filter is simply a series capacitor, the leading edge of the square wave component will pass through unattenuated and the level portion of the square wave component will decrease exponentially in each cycle. The difficulty of removing the undesired components, or transient remanents, by filtering is increased due to the fact that the chrominance components are typically converted to a frequency band of about 687 KHz, but their bandwidth is such that they extend .+-. 500 KHz from the 687 KHz figure. Thus the filter would have to eliminate DC signals but pass all signals between approximately 187 KHz and 1,187 MHz.
It is one of the objects of the present invention to provide an improved system for eliminating the direct voltage offset in a system generally of the foregoing type.
Further objects will be apparent from the following specification together with the drawings.